Solar cell contact and method of making the contact

ABSTRACT

A solar panel comprises a back contact layer, an absorber layer over the back contact layer, a buffer layer over the absorber layer, and a front contact layer comprising a transparent conductive material over the buffer layer. The front contact layer has a plurality of outer edges and a seed layer comprising a seed layer material along the outer edges.

PRIORITY CLAIM AND CROSS-REFERENCE

None.

BACKGROUND

The present disclosure generally relates to photovoltaic solar cells,and more particularly to thin film solar cells and methods for formingsame.

Thin film photovoltaic (PV) solar cells are one class of energy sourcedevices which harness a renewable source of energy in the form of lightthat is converted into useful electrical energy which can be used fornumerous applications. Thin film solar cells are multi-layeredsemiconductor structures formed by depositing various thin layers andfilms of semiconductor and other materials on a substrate. These solarcells can be made into light-weight flexible sheets in some formscomprised of a plurality of individual electrically interconnectedcells. The attributes of light weight and flexibility gives thin filmsolar cells broad potential applicability as an electric power sourcefor use in portable electronics, aerospace, and residential andcommercial buildings where they can be incorporated into variousarchitectural features such as roof shingles, facades, and skylights.

Thin film solar cell semiconductor packages generally include a bottomcontact or electrode formed on the substrate, an absorber, and a topcontact or electrode formed above the bottom contact. Front contactshave been made for example of light transparent conductive oxide (“TCO”)materials. TCO materials are susceptible to attack and degradation byenvironment factors including water, oxygen, and carbon dioxide. SuchTCO degradation may induce high series resistance (Rs) and result inlower solar energy conversions efficiencies for the solar cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features can be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view of a solar panel, in accordance withsome embodiments.

FIG. 2 is a plan view of the solar panel of FIG. 1, in accordance withsome embodiments.

FIG. 3 is a schematic diagram of an apparatus for fabricating the solarpanel of FIG. 1, in accordance with some embodiments.

FIG. 4 is a cross-sectional view of the heating plate shown in FIG. 3,with the solar panel of FIG. 1 thereon, in accordance with someembodiments.

FIG. 5 is a diagram of the temperature of the solar panel of FIG. 4versus time, in accordance with some embodiments.

FIG. 6 is a flow chart of a method of making the solar panel of FIG. 1,using the apparatus of FIG. 3, in accordance with some embodiments.

FIG. 7 is a cross-sectional view of a solar panel, in accordance withsome embodiments.

FIG. 8 is a schematic diagram of an apparatus for fabricating the solarpanel of FIG. 7, with the heating plate in a first position inaccordance with some embodiments.

FIG. 9 shows the apparatus of FIG. 8 with the heating plate in a secondposition in accordance with some embodiments.

FIG. 10 is a diagram of the temperature of the solar panel of FIG. 7versus time, in accordance with some embodiments.

FIG. 11 is a flow chart of a method of making the solar panel of FIG. 7,using the apparatus of FIGS. 8 and 9, in accordance with someembodiments.

FIGS. 12A and 12B are plan and cross-sectional views of the seed layermicrostructure of the solar panel of FIG. 1 or FIG. 7, in accordancewith some embodiments.

FIGS. 12C and 12D are plan and cross-sectional views of the seed layermicrostructure of the solar panel of FIG. 1 or FIG. 7, in accordancewith some embodiments.

FIG. 13 is a diagram of X-ray diffraction (XRD) analysis for the seedlayer material and bulk TCO material shown in FIGS. 12A-12D.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures can be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, can be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus can be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a cross-sectional view of a solar panel 100, in accordancewith some embodiments. FIG. 2 is a plan view of the solar panel of FIG.1, in accordance with some embodiments. The solar panel 100 includes asolar panel substrate 110, a back contact layer 120 on the substrate, anabsorber layer 130 over the back contact layer 120, a buffer layer 140over the absorber layer 130, and a front contact layer 150 comprising abulk transparent conductive material 151 (such as a transparentconductive oxide, or TCO) over the buffer layer 130. The front contactlayer 150 further comprises a seed layer 152 comprising a seed layermaterial along a plurality of outer edges 153 of the solar panel 100.According to some embodiments, the bulk TCO layer 151 and the seed TCOlayer 152 are formed simultaneously in a single process, within a singlechamber. Although a seed layer can be formed over the entire top surfaceof the buffer layer 140, good peeling resistance and good seriesresistance can be achieved by forming the seed layer 152 on the edgeportions 153 of the substrate.

Substrate 110 can include any suitable solar cell substrate material,such as glass. In some embodiments, substrate 110 includes a glasssubstrate, such as soda lime glass, or a flexible metal foil or polymer(e.g., a polyimide, polyethylene terephthalate (PET), polyethylenenaphthalene (PEN) polymeric hydrocarbons, cellulosic polymers,polycarbonates, polyethers, or others.). Other embodiments include stillother substrate materials.

The back contact layer 120 includes any suitable back contact material,such as metal. In some embodiments, back contact layer 120 can includemolybdenum (Mo), platinum (Pt), gold (Au), silver (Ag), nickel (Ni), orcopper (Cu). Other embodiments include still other back contactmaterials. In some embodiments, the back contact layer 120 is from about50 nm to about 2 μm thick. In some embodiments, the back contact layeris formed by sputtering.

The absorber layer 130 includes any suitable absorber material, such asa p-type semiconductor. In some embodiments, the absorber layer 130 caninclude a chalcopyrite-based material comprising, for example,Cu(In,Ga)Se2 (CIGS), cadmium telluride (CdTe), CuInSe2 (CIS), CuGaSe2(CGS), Cu(In,Ga)Se2 (CIGS), Cu(In,Ga)(Se,S)2 (CIGSS), CdTe or amorphoussilicon. Other embodiments include still other absorber materials. Insome embodiments, the absorber layer 130 is from about 0.3 μm to about 3μm thick. The absorber layer 130 can be applied using a variety ofdifferent process. For example, the CIGS precursors can be applied bysputtering. In other embodiments, one or more of the CIGS precursors areapplied by evaporation.

Buffer layer 140 includes any suitable buffer material, such as n-typesemiconductors. In some embodiments, buffer layer 140 can includecadmium sulphide (CdS), zinc sulphide (ZnS), zinc selenide (ZnSe),indium(III) sulfide (In2S3), indium selenide (In2Se3), or Zn1-xMgxO,(e.g., ZnO). Other embodiments include still other buffer materials. Insome embodiments, the buffer layer 140 is from about 1 nm to about 500nm thick. In some embodiments, the buffer layer 140 is applied by a wetprocess, such as chemical bath deposition (CBD).

In some embodiments, front contact layer 150 includes an annealedtransparent conductive oxide (TCO) layer 151. In some embodiments, theTCO layer 151 is highly doped. For example, the charge carrier densityof the TCO layer 151 can be from about 1×10¹⁷ cm-3 to about 1×10¹⁸ cm-3.The TCO material for the annealed TCO layer can include any suitablefront contact material, such as metal oxides and metal oxide precursors.In some embodiments, the TCO material can include zinc oxide (ZnO),cadmium oxide (CdO), indium oxide (In₂O₃), tin dioxide (SnO₂), tantalumpentoxide (Ta₂O₅), gallium indium oxide (GaInO3), (CdSb₂O₃), or indiumoxide (ITO). The TCO material can also be doped with a suitable dopant.In some embodiments, ZnO can be doped with any of aluminum (Al), gallium(Ga), boron (B), indium (In), yttrium (Y), scandium (Sc), fluorine (F),vanadium (V), silicon (Si), germanium (Ge), titanium (Ti), zirconium(Zr), hafnium (Hf), magnesium (Mg), arsenic (As), or hydrogen (H). Inother embodiments, SnO₂ can be doped with antimony (Sb), F, As, niobium(Nb), or tantalum (Ta). In other embodiments, In₂O₃ can be doped withtin (Sn), Mo, Ta, tungsten (W), Zr, F, Ge, Nb, Hf, or Mg. In otherembodiments, CdO can be doped with In or Sn. In other embodiments,GaInO₃ can be doped with Sn or Ge. In other embodiments, CdSb₂O₃ can bedoped with Y. In other embodiments, ITO can be doped with Sn. Otherembodiments include still other TCO materials and corresponding dopants.In some embodiments, the front contact layer 110 is from about 5 nm toabout 3 μm thick. In some embodiments, the front contact layer 151 isformed by metal organic chemical vapor deposition (MOCVD). In otherembodiments, the front contact 151 is formed by sputtering.

In some embodiments a thin film TCO seed layer 152 surrounding a thickerbulk or main TCO front contact layer 151 over the buffer layer 140increases adhesion of the front contact layer 151 to the buffer layer140. Advantageously, the TCO front contact layer 151 is more resistantto peeling damage with the TCO seed layer 152 surrounding the frontcontact layer 151, improving the performance and reliability of thesolar cell 100, particularly when the solar cell undergoes thermalcycling.

In some embodiments, the transparent conductive material of the frontcontact 151 and the seed layer material of the seed layer 152 are bothformed by CVD using the same process gas. In some embodiments, thetransparent conductive material of the front contact 151 and the seedlayer material of the seed layer 152 have the same chemical compositionas each other, and the transparent conductive material and the seedlayer material have different microstructures from each other. Forexample, in some embodiments, the material of seed layer 152 has asmaller grain size than the TCO material of front contact layer 151. Insome embodiments, the seed layer 152 has a crystal orientation differentfrom a crystal orientation of the transparent conductive material.

In some embodiments, the seed layer 152 has a thickness 162 less than athickness 163 of the transparent conductive material of the frontcontact layer 151. For example, in some embodiments, the transparentconductive material of the front contact 151 has a first thickness, theseed layer 152 has a second thickness, and a ratio of the firstthickness to the second thickness is in a range from 2:1 to 30:1.

In some embodiments, the transparent conductive material has a firstwidth 161, the seed layer material has a second width 160 at the outeredges, and a ratio of the first width to the second width is in a rangefrom about 6:1 to about 17:1. Thus, about 80% or more of the total areaof the front contact 150 is occupied by the main TCO material in the TCOlayer 151, and about 20% or less of the total area is occupied by theseed layer 152. In some embodiments, about 95-97% of the total area ofthe front contact 150 is occupied by the main TCO material in the TCOlayer 151, and about 3-5% of the total area is occupied by the seedlayer 152. The material of seed layer 152 has a higher resistivity thanthe material of the main TCO layer 151. Because the seed layer occupiesa relatively small fraction of the total solar panel area, sufficient toavoid TCO peeling, the benefits of the seed layer can be achievedwithout compromising the series resistance (Rs) of the entire solarpanel 100.

In some embodiments, the adhesion improvement and benefits are achievedby forming the TCO seed layer 152 in a deposition process performed atlower temperatures than those used to form the TCO front contact layer151. This produces a seed layer 152 with a different microstructurehaving a finer or smaller grain size than the main TCO front contactlayer 151. The smaller grain size is associated with imparting theincreased adhesion properties to the main TCO layer 151. Accordingly,embodiments of the present disclosure have a TCO seed layer 152 alongthe periphery of the solar panel 100, with a different grain size thanthe main TCO front contact layer 151.

FIG. 3 is a schematic diagram of an apparatus for fabricating the solarpanel 100 of FIG. 1, in accordance with some embodiments. The apparatuscan be a CVD cluster tool or other tool having a process reactionchamber 24 for forming the TCO seed layer 152 and main top electrodelayer 151 over the substrate 110. The process reaction chamber 24includes a process gas supply system 30 which introduces the processgases containing the chemical TCO layer precursors (e.g. withoutlimitation DEZ for formation of ZnO TCO material), dopant in someembodiments for seed layer 152 (optional) and main bulk TCO layer 151,and other process gases into a mixing chamber 32 of reaction chamber 24.Gas flows from the mixing chamber 32 through a header tube 34 into a gasinjection diffuser 36 located at the top of reaction chamber 24.Diffuser 36 (also known by the term “showerhead” in the art) contains aplurality of openings through which gas is uniformly distributedthroughout the reaction chamber. A heating susceptor or heating plate 38is disposed in reaction chamber 24. Heating plate 38 is configured tosupport and heat substrate 110 during the film deposition process.Buffer chamber 22 includes a heating plate 38 and can include an insertgas supply (e.g. nitrogen). In some embodiments, a buffer chamber (notshown) is used for preheating the temperature of the solar cellsubstrate 110 to be processed in the reaction chamber 24 for increasingthe temperature of the substrate from room temperature to approximatelyor just below the process temperature of the substrate to be used in thereaction chamber 24, thereby shortening the process time in the reactionchamber and throughput of the CVD tool.

In some embodiments, for fabricating the seed layer TCO 152, thetemperature of the edges of the substrate should be in a range fromabout 100 degrees C. to about 140 degrees C. In some embodiments forforming the bulk TCO 151, the temperature of the interior of thesubstrate 110 should be from about 150 degrees C. to about 200 degreesC. According to some embodiments described below, the bulk TCO layer 151and the seed layer TCO 152 for a given solar panel 100 are formedsimultaneously in a single process, in a single process chamber 24.

FIG. 4 is a cross-sectional view of the heating plate 38 shown in FIG.3, with the solar panel 110 of FIG. 1 thereon during front contactformation, in accordance with some embodiments. The heating plate 38 hasa top surface for supporting the substrate 110, and for conducting heatto the bottom of the substrate 110. The edges 153 of the substrate 110do not touch the heating plate directly, and thus have a lowertemperature than the center portion of the substrate, when the heatingplate 38 heats the substrate. The substrate conducts some heat from theinterior to the edges 153, so the temperature of the edges rises whenthe heating plate 38 heats the substrate 110, but there is a temperaturedifferential.

The dimensions of the heating plate 38 are smaller than the dimensionsof the substrate 110. The heater plate 38 has a width 37 that is smallerthan a width 39 of the solar panel substrate 110 and adapted to underliethe center of the solar panel substrate 110. In some embodiments, theheating plate 38 has a first width, the substrate extends beyond theheating plate by a second width, and a ratio of the first width to thesecond width is in a range from 6:1 to 17:1. Similarly, the length ofthe heater plate (extending into the page) is smaller than the length ofthe solar panel substrate 110.

In order to form the front contact, the substrate 110 is provided in theprocess chamber 24 at the desired temperature, with the absorber layer130 and buffer layer 140 already formed, and the P2 scribe lines (notshown) already completed. In some embodiments, the TCO layer 150 isformed by heating a solar panel substrate 110 to the desired frontcontact formation temperature in the process chamber 24. In otherembodiments, the solar panel substrate 110 is preheated in a bufferchamber (not shown) and then transferred to the process chamber 24.

FIG. 5 is a diagram of the temperature of the solar panel of FIG. 4versus time, in accordance with some embodiments. As shown in FIG. 5,the temperature at the edges 153 of the substrate (which are not heatedby the plate 38) is lower than the temperature of the interior portionof the substrate 110. In some embodiments, within about 600 seconds, thetemperature of the interior of substrate 110 reaches about 165 degreesC., and the temperature at the edges 153 reaches about 130 degrees C. Atthese respective temperatures, a main TCO front contact layer 151 havinga thickness of 1000 nm to 3000 nm is formed, and the seed layer 152having a thickness of 100 nm to 500 nm is formed. In other embodiments,the desired deposition temperatures for forming the front contact layer151 and seed layer 152 for other TCO materials are determinedexperimentally.

Once the desired substrate process temperatures are reached, the TCOlayer formation process is started by introducing the process gases intoreaction chamber 24. Because of the temperature differential, theinterior of the substrate and the edge portions 153 form crystals of theTCO material with different grain size and orientation. During the sameperiod of time, the thickness of the seed layer 152 deposited on theedges 153 of the substrate by a chemical vapor deposition (CVD) processis less than the thickness of the material 151 deposited over theinterior of the substrate 110. The TCO deposition process continues fora period of time sufficient to form the desired thickness of the bulkTCO layer 151 in the center (interior) portion of the solar panel. Thus,the TCO seed layer 152 has a thickness less than the bulk main TCO topelectrode layer 151. In some embodiments, without limitation, TCO seedlayer 152 has a thickness of about 100 nm to about 500 nm. This issufficient for increasing the adhesion properties of the main TCO topelectrode layer 151 to reduce or eliminate peeling. Simultaneously, TCOtop electrode layer 151 is deposited with a thickness of from about 1000nm to about 3000 nm for low resistance and good current collectionperformance. Accordingly, in some embodiments, TCO seed layer 152 has athickness that is less than half of the main TCO layer 151.

Accordingly, in some embodiments, it is desirable for the thickness ofTCO seed layer 152 to be less than that of TCO top electrode layer 151,since the lower temperature formed seed layer 152 tends to have a higherresistivity than the bulk top electrode layer 151, which inhibitscurrent flow and reduces solar cell performance. The TCO seed layer 152therefore should have a thickness sufficient to improve adhesion of thebulk TCO layer 151 to the absorber layer 130, while not being so thickas to degrade solar cell performance.

When completed, the partially completed thin film solar cell wouldappear as shown in FIG. 1 and described above.

Although formation of the TCO seed layer 152 and top electrode layer 151are described herein with respect to using a CVD process in onenon-limiting example, other suitable film formation processes used inthe semiconductor art can be used including, without limitation atomiclayer deposition (ALD) and physical vapor deposition (PVD) as twoexamples.

An advantage of the foregoing process according to the presentdisclosure is that the TCO seed layer 160 and top electrode layer 150are both formed in the same machine, and are comprised of the samematerial. This creates economies in the solar cell formation fabricationprocess flow and reduces costs.

The process described herein reduces total processing time. Theformation of the seed layer 152 and the main front contact 151 areperformed simultaneously, reducing total deposition time in the chamber.Also, there is no need to incur transfer time moving the substrate 110between a first process chamber for forming layer 152 and a secondprocess chamber for forming layer 151. Further, because both the TCOseed layer 152 and top electrode layer 151 can be formed in a thin filmdeposition tool having a single process reaction chamber 24 without abuffer chamber for preheating the substrate, there is no need to incurtransfer time moving the substrate 110 between a buffer chamber and aprocess chamber.

FIG. 6 is a flow chart of a method of making the solar panel 100 of FIG.1, using the apparatus 24 of FIG. 3, in accordance with someembodiments.

At step 602, the substrate is cleaned. In some embodiments, substrate110 is cleaned by using detergent or chemical in either brushing tool orultrasonic cleaning tool.

At step 604, back electrode layer 120 is then formed on a substrate 110by sputtering, atomic layer deposition (ALD), chemical vapor deposition(CVD), or other suitable techniques.

At step 606, the P1 patterned scribe lines (not shown) are next formedin bottom electrode layer 120 to expose the top surface of substrate 110as shown. Any suitable scribing method can be used such as, withoutlimitation, mechanical scribing with a stylus or laser scribing.

At step 608, the p-type doped semiconductor light absorber layer 130 isnext formed on top of bottom electrode layer 120. The absorber layer 130material further fills the P1 scribe line and contacts the exposed topsurface of substrate 110 to interconnect layer 130 to the substrate.Absorber layer 130 formed of CIGS can be formed by any suitable vacuumor non-vacuum process. Such processes include, without limitation,selenization, sulfurization after selenization (“SAS”), evaporation,sputtering electrodeposition, chemical vapor deposition, or ink sprayingor the like.

At step 610, an n-type buffer layer 140, which can be CdS for example,is then formed on absorber layer 130 to create an electrically activen-p junction. Buffer layer 140 can be formed by an electrolyte chemicalbath deposition (CBD) process for forming such layers using anelectrolyte solution that contains sulfur.

At step 612, the P2 scribe lines (not shown) are next cut through theabsorber layer 130 to expose the top surface of the bottom electrode 120within the open scribe line or channel. Any suitable method can be usedto cut the P2 scribe line, including without limitation mechanical (e.g.cutting stylus) or laser scribing. The P2 scribe line will subsequentlybe filled with a conductive material from top electrode layer 150 tointerconnect the top electrode to the bottom electrode layer 120.

At step 614, the front contact 150, including the seed layer 152 and themain front contact (bulk TCO) 151 are formed, in the manner describedabove with reference to FIGS. 1-3. The step of forming the front contact150 includes heating a center portion of the substrate 110 using aheating plate 38 that is smaller in a width dimension than the solarcell substrate 110. A center portion of the substrate 110 is heatedwithout directly applying heat to the outer edges 110 e of the solarpanel. The center portion of the substrate 110 is heated to a range ofabout 150 degrees C. to about 200 degrees C., while the outer edges arein a temperature range from about 100 degrees C. to about 140 degrees C.The process gas is supplied, and thin films are formed with respectivelydifferent grain structures on the interior and edges of the substrate,due to the temperature differential. As a result, the transparentconductive material and the seed layer material are applied on thebuffer layer simultaneously. The top electrode 150 is thus configured tocarry the collected charge to an external circuit. The P2 scribe line isalso at least partially filled with the TCO material to form anelectrical connection between the top electrode layer 150 of one solarcell and the bottom electrode 120 of the adjacent solar cell within thesolar panel 100, creating an electron flow path.

At step 616, following formation of the TCO seed layer 152 and topelectrode layer 151 described above, the P3 scribe line (not shown) isformed. The P3 scribe line extends through (from top to bottom) TCO topelectrode layer 150, buffer layer 140, absorber layer 130, and thebottom electrode layer 120 down to the top of substrate 110.

At step 618, a combination of ethylene vinyl acetate (EVA) and butyl areapplied to seal the solar panel 100. The EVA and butyl encapsulant isapplied directly onto the top electrode layer 150 in some embodiments.The EVA/butyl act as a suitable light transmitting encapsulant.

At step 620, heat and pressure are applied to laminate the EVA/butylfilm to the front contact 150.

At step 622, additional back end of line processes can be performed.This can include laminating a top cover glass onto solar cell structureto protect the top electrode layer 150.

At step 624, suitable further back end processes can then be completed,which can include forming front conductive grid contacts and one or moreanti-reflective coatings (not shown) above top electrode 150. The gridcontacts protrude upwards through and beyond the top surface of anyanti-reflective coatings for connection to external circuits. The solarcell fabrication process produces a finished and complete thin filmsolar cell module 100.

FIG. 7 is a cross-sectional view of a solar panel 200, in accordancewith some embodiments. The solar panel 200 includes a solar panelsubstrate 110, a back contact layer 120 on the substrate, an absorberlayer 130 over the back contact layer 120, a buffer layer 140 over theabsorber layer 130, and a front contact layer 250 comprising a bulktransparent conductive material 251 (such as a transparent conductiveoxide, or TCO) over the buffer layer 130. The substrate 110, backcontact 120, absorber 130, and buffer layer 140 can be the same asdescribed above with reference to FIGS. 1-6. The front contact layer 250comprises a seed layer 252 comprising a seed layer material along aplurality of outer edges 153 of the solar panel 200. The main frontcontact layer 251 further comprises an edge layer 251 e of thetransparent conductive material extending out to the edges of the solarpanel 200, overlying the seed layer 252 at the edges of the substrate.According to some embodiments, the bulk TCO layer 251 and the seed TCOlayer 252 are formed in a single process, within a single chamber.

Because the edges of the solar panel 200 have the TCO edge layer 251 eoverlying the seed layer 252, a parallel conductive path is provided tofurther reduce the series resistance of the TCO layer 250.

FIG. 8 is a schematic diagram of an apparatus for fabricating the solarpanel of FIG. 7. The apparatus includes an actuator 240 configured toraise and lower one of the group consisting of a first heating plate 238and a second heating plate 237 relative to the other of the groupconsisting of the first heating plate and the second heating plate. Inthe example, the first heating plate 238 is movable, and the secondheating plate 237 is fixed, but in other embodiments (not shown) thefirst heating plate 238 is fixed and the second heating plate 237 can belowered. FIG. 8 shows the apparatus with the heating plate 238 in afirst position in accordance with some embodiments. FIG. 9 shows theapparatus of FIG. 8 with the heating plate in a second position.

The apparatus includes a first heater plate 238 within the chamber 24.The first heater plate 238 has a width that is smaller than a width ofthe solar panel substrate 110 and adapted to underlie the center(interior portion) of the solar panel substrate 110.

A second heater plate 237 is adjacent the first heater plate 238. Thesecond heater plate 237 is positioned to underlie the edges 110 e of thesubstrate 110, such that the first heater plate 238 and second heaterplate 237 together underlie the center and edges 110 e of the solarpanel substrate 110.

In some embodiments, the first heating plate 238 has a first width, thesecond heating plate 237 has a second width extending from an inner edgethereof to an outer edge thereof, and a ratio of the first width to thesecond width is in a range from 6:1 to 17:1. In FIG. 9, the secondheating plate 237 has an outer edge that extends slightly beyond theouter edge of the substrate, and the width of the second heating plateis slightly larger than the width of the substrate. In other embodiments(not shown), the width of the second heating plate can be the same asthe width of the substrate 110.

The apparatus is operable in a first mode (FIG. 8) for directly heatingthe center of the solar panel substrate 110 but not the edges 110 e, andthe apparatus is operable in a second mode (FIG. 9) for directly heatingthe center and edges 110 e of the solar panel substrate 110.

FIG. 10 is a diagram of a temperature profile to be achieved bysuccessively actuating the first heating plate 238 to the raised andlowered positions. Curve 1004 shows a desired temperature profile forthe seed layer. By about 750 seconds after heating begins, the edges 110e have a temperature of 130 degrees C. for forming the seed layer 152,and the center (interior) of the substrate has a temperature of about165 degrees C. for forming the main front contact TCO 151. At about time1500, the temperature at the edges 110 e is increased up to about thesame temperature as the interior of the solar panel substrate 110, sothat the edge layer 251 e of bulk TCO having the larger grain size (withlower resistance) is formed above the seed layer 252.

FIG. 11 is a flow chart of a method of making the solar panel of FIG. 7,using the apparatus of FIGS. 8 and 9, in accordance with someembodiments.

At step 1102, the substrate is cleaned. In some embodiments, substrate110 is cleaned by using detergent or chemical in either brushing tool orultrasonic cleaning tool.

At step 1104, back electrode layer 120 is then formed on a substrate 110by sputtering, atomic layer deposition (ALD), chemical vapor deposition(CVD), or other suitable techniques.

At step 1106, the P1 patterned scribe lines (not shown) are next formedin bottom electrode layer 120 to expose the top surface of substrate 110as shown. Any suitable scribing method can be used such as, withoutlimitation, mechanical scribing with a stylus or laser scribing.

At step 1108, the p-type doped semiconductor light absorber layer 130 isnext formed on top of bottom electrode layer 120. The absorber layer 130material further fills the P1 scribe line and contacts the exposed topsurface of substrate 110 to interconnect layer 130 to the substrate.Absorber layer 130 formed of CIGS can be formed by any suitable vacuumor non-vacuum process. Such processes include, without limitation,selenization, sulfurization after selenization (“SAS”), evaporation,sputtering electrodeposition, chemical vapor deposition, or ink sprayingor the like.

At step 1110, an n-type buffer layer 140, which can be CdS for example,is then formed on absorber layer 130 to create an electrically activen-p junction. Buffer layer 140 can be formed by an electrolyte chemicalbath deposition (CBD) process for forming such layers using anelectrolyte solution that contains sulfur.

At step 1112, the P2 scribe lines (not shown) are next cut through theabsorber layer 130 to expose the top surface of the bottom electrode 120within the open scribe line or channel. Any suitable method can be usedto cut the P2 scribe line, including without limitation mechanical (e.g.cutting stylus) or laser scribing. The P2 scribe line will subsequentlybe filled with a conductive material from top electrode layer 150 tointerconnect the top electrode to the bottom electrode layer 120.

At step 1114, the controller 241 causes the actuator 240 to raise thefirst heating plate 238 that is smaller in a width dimension than thesubstrate 110, while the substrate is on the first heating plate.

At step 1116, the transparent conductive material is applied over thebuffer layer while heating a center portion of the substrate 110 using aheating plate 38 that is smaller in a width dimension than the solarcell substrate 110. A center portion of the substrate 110 is heatedwithout directly applying heat to the outer edges 110 e of the solarpanel 110. The center portion of the substrate 110 is heated to a rangeof about 150 degrees C. to about 200 degrees C., while the outer edgesare in a temperature range from about 100 degrees C. to about 140degrees C. The process gas is supplied, and thin films are formed withrespectively different grain structures on the interior and edges of thesubstrate, due to the temperature differential. As a result, thetransparent conductive material and the seed layer material are appliedon the buffer layer simultaneously.

At step 1118, the first heating plate 238 is lowered until a top surfaceof the first heating plate 238 is flush with a top surface of a secondheating plate 237 under the edge 110 e of the substrate. From this timeonward, the center portion of the substrate 110 and the edges 110 e ofthe substrate are both heated by the first heating plate 238 and secondheating plate 237, respectively.

At step 1120, additional process gas (TCO material) is applied whileheating the center of the substrate 110 with the first heating plate 238and heating the edge 110 e of the substrate with the second heatingplate 237 simultaneously.

The top electrode 150 is thus configured to carry the collected chargeto an external circuit. The P2 scribe line is also at least partiallyfilled with the TCO material to form an electrical connection betweenthe top electrode layer 150 of one solar cell and the bottom electrode120 of the adjacent solar cell within the solar panel 100, creating anelectron flow path.

At step 1122, following formation of the TCO seed layer 152 and topelectrode layer 151 described above, the P3 scribe line (not shown) isformed. The P3 scribe line extends through (from top to bottom) TCO topelectrode layer 150, buffer layer 140, absorber layer 130, and thebottom electrode layer 120 down to the top of substrate 110.

At step 1124, a combination of ethylene vinyl acetate (EVA) and butylare applied to seal the solar panel 100. The EVA and butyl encapsulantis applied directly onto the top electrode layer 150 in someembodiments. The EVA/butyl act as a suitable light transmittingencapsulant.

At step 1126, heat and pressure are applied to laminate the EVA/butylfilm to the front contact 150.

At step 1128, additional back end of line processes can be performed.This can include laminating a top cover glass onto solar cell structureto protect the top electrode layer 150.

At step 1130, suitable further back end processes can then be completed,which can include forming front conductive grid contacts and one or moreanti-reflective coatings (not shown) above top electrode 150. The gridcontacts protrude upwards through and beyond the top surface of anyanti-reflective coatings for connection to external circuits. The solarcell fabrication process produces a finished and complete thin filmsolar cell module 100.

FIGS. 12A and 12B are scanning electron microscope (SEM) images showingthe microstructure of seed layer 152, and FIGS. 12C and 12D are SEMimages showing the microstructure of the higher temperature formed bulkTCO layer 151. Compared to the TCO bulk layer 151 (FIGS. 12C, 12D)formed at higher deposition temperatures, the smaller grain size of theseed layer 152 (FIGS. 12A, 12B) polycrystalline structure associatedwith improving the adhesive property of the TCO top electrode layer 150is evident. FIG. 13 shows an X-ray diffraction (XRD) analysis of the TCOseed layer material and bulk top electrode material. FIG. 13 is a plotof reflected intensities versus the detector angle of the XRD analysiswhich shows that the TCO seed layer polycrystalline structure hascrystals with a different orientation angle of about 34.4 degrees incontrast to the bulk TCO layer material with an angle of about 32degrees, thereby further confirming the different crystallineorientation and grain structure of the seed layer material. Thedifferent structure of the TCO seed layer material and adhesionproperties are achieved through the lower CVD deposition temperaturesused according to the present disclosure.

Some embodiments described herein form a TCO seed layer and main TCOcontact layer in the same process, without transferring the solar panelsubstrate between tools, and without breaking vacuum. Transport delaysare eliminated and process time can be shortened. By providing a TCOseed layer at the edges, but not on the interior of the substrate,peeling is reduced or avoided, without degrading Rs.

In some embodiments, a solar panel comprises a back contact layer, anabsorber layer over the back contact layer, a buffer layer over theabsorber layer, and a front contact layer comprising a transparentconductive material over the buffer layer. The front contact layer has aplurality of outer edges and a seed layer comprising a seed layermaterial along the outer edges.

In some embodiments, a method of making a solar panel comprises: forminga back contact; forming an absorber layer over the back contact; forminga buffer layer over the absorber layer, and forming a front contactcomprising a transparent conductive material over the buffer layer, thefront contact having a plurality of outer edges and a seed layercomprising a seed layer material along the outer edges.

In some embodiments, an apparatus, comprises a chamber having one ormore openings for supplying a process gas for forming a transparentconductive layer over a solar panel substrate having a center and aplurality of edges. A first heater plate is within the chamber. Thefirst heater plate has a width that is smaller than a width of the solarpanel substrate and adapted to underlie the center of the solar panelsubstrate. A second heater plate is adjacent the first heater plate,positioned to underlie the edges, such that the first heater plate andsecond heater plate together underlie the center and edges of the solarpanel substrate. The apparatus is operable in a first mode for directlyheating the center of the solar panel substrate but not the edges. Theapparatus is operable in a second mode for directly heating the centerand edges of the solar panel substrate.

The foregoing outlines features of several embodiments so that thoseskilled in the art can better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1-8. (canceled)
 9. A method of making a solar panel, comprising: forminga back contact; forming an absorber layer over the back contact; forminga buffer layer over the absorber layer, and forming a front contactcomprising a transparent conductive material over the buffer layer, thefront contact having a plurality of outer edges and a seed layercomprising a seed layer material along the outer edges.
 10. The methodof claim 9, wherein the forming step includes applying the transparentconductive material and the seed layer material on the buffer layersimultaneously.
 11. The method of claim 10, wherein the back contact isformed above a substrate, and the step of forming the front contactincludes heating a center portion of the substrate without directlyapplying heat to the outer edges of the solar panel.
 12. The method ofclaim 10, wherein the step of forming the front contact includes heatinga center portion of the substrate to a range of about 150 degrees C. toabout 200 degrees C., while the outer edges are in a temperature rangefrom about 100 degrees C. to about 140 degrees C.
 13. The method ofclaim 10, wherein the step of forming the front contact includes heatinga center portion of the substrate using a heating plate that is smallerin a width dimension that the solar cell substrate.
 14. The method ofclaim 13, wherein the back contact is formed above a substrate, and thestep of forming the front contact further comprises: heating the centerportion of the substrate and the edges of the substrate after using aheating plate that is smaller in a width dimension than the substrate.15. The method of claim 9, wherein the back contact is formed above asubstrate, and the step of forming the front contact comprises: raisinga first heating plate that is smaller in a width dimension than thesubstrate, while the substrate is on the first heating plate; applyingthe transparent conductive material over the buffer layer while heatinga center portion of the substrate; lowering the first heating plateuntil a top surface of the first heating plate is flush with a topsurface of a second heating plate under the edge of the substrate; andapplying additional transparent conductive material while heating thecenter of the substrate with the first heating plate and heating theedge of the substrate with the second heating plate simultaneously.16-20. (canceled)
 21. A method of making a solar panel, comprising:forming a back contact; forming an absorber layer over the back contact;forming a buffer layer over the absorber layer, the buffer layer havingan interior region and a plurality of outer edges around the interiorregion; and forming a front contact comprising a transparent conductivematerial on an interior region of the buffer layer, and a seed layercomprising a seed layer material on the outer edges of the buffer layer.22. The method of claim 21, wherein: the transparent conductive materialand the seed layer material have the same chemical composition as eachother; and the seed layer material has a smaller grain size than thetransparent conductive material.
 23. The method of claim 22, wherein theforming step includes applying the transparent conductive material andthe seed layer material on the buffer layer simultaneously.
 24. Themethod of claim 21, wherein the transparent conductive material has afirst thickness, the seed layer has a second thickness, and a ratio ofthe first thickness to the second thickness is in a range from 2:1 to30:1.
 25. The method of claim 24, wherein the back contact is formedabove a substrate, and the step of forming the front contact includesheating a center portion of the substrate without directly applying heatto the outer edges of the solar panel.
 26. The method of claim 25,wherein the step of forming the front contact includes heating thecenter portion of the substrate to a range of about 150 degrees C. toabout 200 degrees C., while the outer edges are in a temperature rangefrom about 100 degrees C. to about 140 degrees C.
 27. The method ofclaim 25, wherein the step of forming the front contact includes heatinga center portion of the substrate using a heating plate that is smallerin a width dimension that the solar cell substrate.
 28. The method ofclaim 21, wherein the seed layer material has a crystal orientation ofabout 34.4 degrees, and the transparent conductive material has acrystal orientation of about 32 degrees.
 29. A method of making a solarpanel, comprising: forming a back contact; forming an absorber layerover the back contact; forming a buffer layer over the absorber layer,the buffer layer having an interior region and a plurality of outeredges around the interior region; and simultaneously forming atransparent conductive material on an interior region of the bufferlayer and a seed layer comprising a seed layer material on the outeredges of the buffer layer, the seed layer material having a samechemical composition as the transparent conductive material and asmaller grain size than the transparent conductive material.
 30. Themethod of claim 29, wherein the back contact is formed above asubstrate, and the step of forming the front contact includes heating acenter portion of the substrate without directly applying heat to theouter edges of the solar panel.
 31. The method of claim 30, wherein thestep of simultaneously forming includes heating a center portion of thesubstrate using a heating plate that is smaller in a width dimensionthat the solar cell substrate.
 32. The method of claim 31, wherein theback contact is formed above a substrate, and the step of simultaneouslyforming further comprises: heating the center portion of the substrateand the edges of the substrate after using a heating plate that issmaller in a width dimension than the substrate
 33. The method of claim29, wherein the back contact is formed above a substrate, and the stepof simultaneously forming comprises: raising a first heating plate thatis smaller in a width dimension than the substrate, while the substrateis on the first heating plate; applying the transparent conductivematerial over the buffer layer while heating a center portion of thesubstrate; lowering the first heating plate until a top surface of thefirst heating plate is flush with a top surface of a second heatingplate under the edge of the substrate; and applying additionaltransparent conductive material while heating the center of thesubstrate with the first heating plate and heating the edge of thesubstrate with the second heating plate simultaneously.